This invention relates to methanol synthesis. Methanol is conventionally synthesized at elevated and pressure in a methanol synthesis loop where synthesis gas, containing hydrogen, carbon oxides, and, usually, some inerts such as nitrogen and methane, is passed over a copper catalyst at an elevated temperature, typically 200-300xc2x0 C., and pressure, typically 40-150 bar abs., and then the product reacted gas is cooled, condensed methanol is separated and the unreacted gas is recycled to the synthesis reactor. Fresh synthesis gas, hereinafter termed make-up gas, is added to the loop at a suitable location, usually to the recycled unreacted gas before the latter is fed to the synthesis reactor. A purge is taken from the loop at a suitable point to avoid the build-up of inerts to an uneconomically high level. The make-up gas may be added to the loop before or after the separation step.
Methanol synthesis is an exothermic process and it is necessary to limit the amount of reaction occurring in a bed of catalyst and/or to cool the bed, to avoid overheating the catalyst. To this end, a variety of reactor types have been employed. For example, it has been proposed to employ a reactor with means to inject cool quench gas (generally a mixture of make-up gas and unreacted recycle gas) into the catalyst bed or between beds. Examples of such quench bed reactors are described in GB 1105614, EP 0297474, EP 0359952 and U.S. Pat. No. 4,859,425. It has also been proposed to employ reactors having heat exchangers within the beds so that heat evolved by the reaction is transferred to a coolant. Thus, in the arrangement described in U.S. Pat. No. 4,778,662 the synthesis reactor has coolant tubes which extend through at least the inlet part of the catalyst bed and open into the space above the inlet to the catalyst bed: the coolant is the mixture of recycled unreacted gas and make-up gas so that the reactants are heated to the desired inlet temperature by the evolved heat. In the arrangement described in GB 2046618 the catalyst is disposed as a single bed through which the reactants flow radially and heat exchange tubes are provided through which a coolant, e.g. pressurized boiling water, is circulated.
It is often desirable to increase the amount of methanol synthesized. In U.S. Pat. No. 5,252,609 and U.S. Pat. No. 5,631,302 methods are described wherein the make-up gas is subjected to a preliminary synthesis step before it is added to the synthesis loop. In EP 0790226 an arrangement is described where there are two synthesis reactors in series in the loop; the first reactor being cooled by heat exchange with boiling water while the second is cooled by heat exchange with the mixture of make-up gas and recycled unreacted gas.
The throughput may also be increased by operating the loop at a lower circulation ratio, which is defined herein as the ratio of the flow rate of the gas recycled from the separator to the rate at which make-up gas is fed to the loop. In a conventional methanol synthesis process, this circulation ratio is generally in the range 3 to 7. In the present invention, low circulation ratios may be employed, generally in the range 1 to 4, particularly 1 to 3, and preferably below 2.5, especially below 2. However the use of a preliminary synthesis step, or operation at low circulation ratios, has the problem that the partial pressures of the reactants of the gas fed to the preliminary synthesis step, or to the first synthesis stage of the loop, may be relatively high leading to excessive reaction, and excessive heat evolution in the catalyst bed.
We have devised a method whereby this problem may be overcome. In the present invention, methanol synthesis is effected in one or more synthesis stages from recycled unreacted gas, to which part of the make-up gas may have been added, and then the remainder of the make-up gas is added and the mixture passed through one or more further synthesis stages with at least the final synthesis stage of the loop being effected in indirect heat exchange with pressurized water as a coolant.
According to the present invention we provide a process wherein methanol is synthesized in a synthesis loop from a synthesis gas mixture comprising hydrogen and carbon oxides in at least two synthesis stages, characterised in that methanol is synthesized from recycled unreacted gas, optionally together with part of the make-up gas, in one or more synthesis stages to give a stream of reacted gas, make-up gas is then added and, prior to separation of the synthesized methanol, a further amount of methanol is synthesized from the resultant mixture in one or more further synthesis stages, with at least the final synthesis stage of the loop being effected in indirect heat exchange with pressurized water as a coolant.
In its simplest form the synthesis loop has two stages of methanol synthesis with make-up gas being added between the stages and at least the final synthesis stage of the loop is effected in indirect heat exchange with pressurised water as a coolant. The reactor used for synthesis in indirect heat exchange with pressurised water is herein termed a water-cooled reactor.
The first stage is preferably effected in a quench reactor or a heat exchange reactor wherein the synthesis catalyst is cooled by transferring heat evolved by the synthesis reaction by heat exchange to the feed gas of that reactor, e.g. as described in the aforesaid U.S. Pat. No. 4,778,662. Where more than two stages are employed, it is again preferred that the first stage is effected in a quench reactor or a heat exchange reactor as aforesaid and at least the last of the subsequent stage or stages is effected in the water-cooled reactor.
It is preferred that at least 5% of the make-up gas is added to the recycled unreacted gas before the latter is fed to the first synthesis stage. It is preferred that at least 10%, particularly at least 30%, of the make-up gas is added to the loop after the first synthesis stage, especially if the circulation rate is low, e.g. below 2. The proportion of the make-up gas that is added to the loop after the first synthesis stage will depend upon the type of reactor employed for the first synthesis stage and on the circulation ratio.
The first synthesis stage is preferably effected adiabatically.
Thus in one form of the invention, the first stage employs a quench reactor wherein some or all of the recycled unreacted gas, optionally to which part of the make-up gas has been added, is fed to the inlet and the remainder of the recycled unreacted gas, optionally in admixture with some of the make-up gas is used as the quench gas. The remainder of the make-up gas is added to the gas from the outlet of the quench reactor and the mixture is then fed to the water-cooled reactor.
Where a quench reactor is employed for the first synthesis stage, typically only about 20-25% of the recycled unreacted gas is fed to the quench reactor inlet: the balance, to which make-up gas may be added, is used as the quench gas. The quench reactor may have several beds of synthesis catalyst with injection of quench gas between each bed. With such a reactor it is preferred that at least 50% of the make-up gas is added to the reacted gas from the quench reactor after the first synthesis stage, i.e. before it is fed to the water-cooled reactor and, optionally, as part or all of the quench gas.
Where a heat exchange reactor, e.g. of the type described in U.S. Pat. No. 4,778,662, wherein the catalyst is cooled by transferring heat evolved by the synthesis reaction by heat exchange to the feed gas to that reactor, is employed for the first stage, a larger proportion, for example 30 to 90%, particularly 40 to 70%, of the make-up gas may be added to the recycled unreacted gas before the latter is fed to the first synthesis stage. After leaving the first synthesis stage, the remainder of the make-up gas is added and the mixture passed through one or more further catalyst beds, disposed in the water-cooled reactor.
The water-cooled reactor may have the catalyst disposed in tubes with the pressurized water circulating past the exterior of the tubes. However it is preferred that the catalyst is disposed as a single bed with the pressurized water passing through cooling tubes disposed within the catalyst bed.
The make-up gas is often produced by a steam reforming process wherein a hydrocarbon feedstock, such as natural gas, is reacted with steam at an elevated pressure, e.g. in the range 20 to 80 bar abs., and at an elevated temperature, e.g. in the range 700 to 1100xc2x0 C., in the presence of a catalyst. This reforming reaction is strongly endothermic and at least part of the reforming reaction is generally operated with the catalyst disposed in tubes through which the feedstock/steam mixture passes while the tubes are heated externally by a suitable medium.
In the present invention, the heated pressurized water from the water-cooled reactor may be employed to supply at least part of the steam required for making the make-up gas. Thus the heated pressurized water, preferably after further heating, is directly contacted with the hydrocarbon feedstock before the latter is subjected to the reforming reaction. Such direct contact of the hydrocarbon feedstock with hot water is herein termed saturation. It will be appreciated that since the water is contacted directly with the hydrocarbon feedstock, the pressure of the pressurized water is equal to or greater than that employed in the reforming reaction. Normally, the feedstock, e.g. natural gas, at an elevated pressure is subjected to desulphurisation prior to reforming. It is generally desirable to effect the contacting with the pressurized water after any such desulphurisation step.
In a preferred arrangement, the reforming is effected in two stages. In the first, primary reforming, stage the feedstock/steam mixture is passed over a steam reforming catalyst, usually nickel supported on an inert support, e.g. alumina or a calcium aluminate cement, disposed in externally heated tubes. In the second stage, the primary reformed gas mixture is subjected to a secondary reforming stage wherein it is partially combusted with oxygen and passed through a secondary reforming catalyst. The secondary reforming catalyst is normally disposed as a single bed, again usually of nickel supported on an inert support, e.g. alumina or a calcium aluminate cement. By adjusting the amount of oxygen employed relative to the amount of feedstock, a secondary reformed gas that approximates to the stoichiometric composition for methanol synthesis may be obtained. If the secondary reforming stage is omitted, the reformed gas is liable to have an excess of hydrogen over that required for methanol synthesis, especially where the feedstock is natural gas. In a preferred version of a reforming process employing primary and secondary reforming, the primary reforming is effected in a heat exchange reformer with the heating required for the primary reforming stage being provided by passing the secondary reformed gas past the tubes containing the primary reforming catalyst.
The reformed gas is cooled and excess steam condensed therefrom before compression, if any, of the reformed gas to the synthesis loop pressure. The cooling of the reformed gas preferably includes further heating of the pressurized water before the latter is contacted with the hydrocarbon feedstock. It may also include other heat recovery, e.g. heating of pressurized water fed to the synthesis reactor, and the provision of heat for distillation of product methanol.